1. Field of the Invention
The present invention relates to an imaging apparatus and an imaging method. More particularly, the present invention relates to an imaging apparatus and an imaging method having a camera shake correcting function and a photographing sensitivity changing function.
2. Description of Related Art
Imaging apparatuses such as digital still cameras and digital video cameras that convert an optical image of the photographing object to electrical image signal and outputs image signal (hereinafter simply referred to as “digital cameras”), have become popular. With reductions in size and weight and escalation in the magnification rate of optical zooming in recent years in particular, digital cameras have become convenient for photographers.
However, accompanying reductions in size and weight and escalation in the magnification rate of optical zooming of digital cameras, a blur may occur in photographed images and may cause image quality degradation.
Patent Document 1 (Japanese Patent Application Laid-Open No. 2000-13671) discloses a digital camera with a blur correcting optical system that reduces the influence of image shake upon the image when a photograph is taken. The digital camera disclosed in Patent Document 1 moves the correction lens up, down, left and right in directions perpendicular to the optical axis, depending on image shake of when a photograph is taken, and corrects image distortion. By this means, it is possible to take a photograph with reduced image shake using a smaller-sized and lighter-weighted digital camera. Furthermore, the digital camera disclosed in Patent Document 1 does not have to use a flash lamp to emit light upon taking a photograph to prevent image shake, so that it is possible to take a photograph under conditions producing similar atmosphere to natural colors.
On the other hand, among causes for degrading image quality of photographed images is object shake caused by the motion of the photographing object, in addition to camera shake caused by vibration such as caused by a shaking hand, added to the camera. Object shake can be prevented by making exposure time shorter and taking a photograph at a high shutter speed. Shutter speed can be made faster by, for example, increasing photographing sensitivity or by lighting a flash lamp. As for optical image shake of the photographing object in the imaging plane, shake caused by vibration applied to the camera will be referred to as “camera shake” and shake caused by the motion of the photographing object will be referred to as “object shake.” Camera shake and object shake will be collectively referred to as “image shake” with respect to the imaging plane.
Patent Document 2 (Japanese Patent Application Laid-Open No. 2006-157428) discloses an apparatus with a motion prediction section for predicting the motion of the photographing object and changing photographing conditions such as shutter speed when the photographing object is likely to move, and a method applicable with the apparatus.
Patent Document 3 (Japanese Patent Application Laid-Open No. 2003-107335) discloses a technique of detecting the face, eyes, nose and mouth of a person in image data, using part of the detected face of the person as the automatic focus area (hereinafter referred to as “AF area”) and performing automatic focus control.
Generally, when photographing sensitivity is increased, the output signal from the imaging sensor is amplified, and, consequently, noise generated from the imaging sensor is also amplified. Therefore, an image taken in high sensitivity contains a large amount of noise. Increasing photographing sensitivity more than necessary may thus result in image quality degradation. It is therefore desirable to increase photographing sensitivity when camera shake still occurs due to insufficient ambient brightness after correction by the correcting optical system or when a fast-moving photographing object is photographed.
However, with such a conventional imaging apparatus, it is difficult for photographers to identify what level of moving speed of the photographing object causes object shake. Therefore, cases often occur where even though it is possible to take a photograph without object shake, the photographer observing the motion of the photographing object misjudges that object shake will occur. As a result, there is a problem that the photographers change photographing sensitivity to high sensitivity and take a photograph containing a large amount of noise. Furthermore, there is a problem that photographers need to change photographing sensitivity immediately before taking a photograph and might miss the chance to take a photograph.
That is, a general photographer cannot identify what level of moving speed of the photographing object will or will not cause object shake. In other words, using the camera shake correcting function may result in taking a photograph with object shake when the photographing object is moving fast, and increasing ISO sensitivity may result in taking a photograph with a large amount of noise when the photographing object is moving slowly. Therefore, taking photographs in good quality is not possible.
Furthermore, although the digital camera having a blur correcting optical system disclosed in Patent Document 1 can reduce image quality degradation due to camera shake, there is no proposal of easing image quality degradation caused by object shake.
Furthermore, since the digital camera disclosed in Patent Document 2 is only directed to predicting the motion of the photographing object and is not directed to deciding what level of moving speed of the photographing object will or will not cause object shake, it is not always possible to take a photograph at an optimal shutter speed matching the speed of the photographing object.